Golf balls are formed from a variety of compositions, which provides a golf ball manufacturer the ability to alter feel and aerodynamic characteristics of a particular ball. For example, golf ball covers formed from balata allow a highly skilled golfer to achieve spin rates sufficient to more precisely control ball direction and distance, particularly on shorter shots. Balata covered golf balls are easily damaged, however, which discourages the average golfer from using such balls. To remedy this durability issue, manufacturers typically use ionomer resin as a cover material. However, while ionomer resin covered golf balls possess virtually cut-proof covers, the spin and feel are inferior compared to balata covered balls.
Polyurethanes and polyureas have also been recognized as useful materials for golf ball covers since the resulting golf balls are durable like ionomer resin, but have the soft feel of a balata covered golf ball. U.S. Pat. No. 4,123,061 teaches a golf ball made from a polyurethane prepolymer formed of polyether with diisocyanate that is cured with either a polyol or an amine-type curing agent. In addition, U.S. Pat. No. 5,334,673 discloses the use of two categories of polyurethane available on the market, i.e., thermoset and thermoplastic polyurethanes, for forming golf ball covers and, in particular, thermoset polyurethane covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent, and/or a difunctional glycol. U.S. Pat. No. 5,484,870 discloses a polyurea composition comprising the reaction product of an organic diisocyanate and an organic amine, each having at least two functional groups. Once these two ingredients are combined, the polyurea is formed and, thus, the ability to vary the physical properties of the composition is limited.
While polyurethane and polyurea covered golf balls are softer than ionomer resin covered golf balls, such balls do not fully match ionomer resin golf balls with respect to resilience or the rebound of the golf ball cover, which is a function of the initial velocity of a golf ball after impact with a golf club. Furthermore, because the polyurethanes and polyureas used to make the covers of such golf balls generally contain an aromatic component, e.g., aromatic diisocyanate, polyol, or polyamine, they are susceptible to discoloration upon exposure to light, particularly ultraviolet (UV) light. To slow down the discoloration, light and UV stabilizers, e.g., Tinuvin 770, 765, and 328, are added to these aromatic polymeric materials. However, to further ensure that the covers formed from aromatic polyurethanes do not appear discolored, the covers are painted with white paint and then covered with a clear coat to maintain the white color of the golf ball. The application of a uniform white pigmented coat to the dimpled surface of the golf ball is a difficult process that adds time and costs to the manufacture of a golf ball.
In addition, the curing agents typically used in polyurethane compostions have relatively high freezing points, which make shipping and storage of these materials during the winter season problematic. This problem is compounded when the compositions include pigments. For example, when certain curatives have thawed from a frozen state, the solids separate and the quality of the pigment dispersed therein can be lost, as measured by the Hegman scale. The Hegman scale is a measurement of particle size, which is-typically used to denote the degree of pigment dispersion. When a material completely loses its quality of dispersion (Hegman equals 0), the particle size of the material is generally about 100 microns or greater. For example, a curative that contains pigment, such as 1,4-butanediol, loses pigment dispersion quality upon freezing. To overcome this loss of pigment dispersion, the separated blend would need to be redispersed before being incorporated into various compositions. In cases where the pigment has undergone “hard settling,” however, the pigment cannot be redispersed and the blend is rendered unusable.
There remains a continuing need for improved compositions that are easily processed into golf balls having performance characteristics, improved resilience, increased cut, scratch and abrasion resistance, enhanced adherence, and improved light stability. Thus, there also remains a need for improved curative blends, particularly those in which the pigment is dispersed, that are able to withstand lower temperature storage and shipping conditions. In addition, it would be advantageous to provide such agents that, even if frozen, are still able to be used without redispersing the pigment or without sacrificing properties of the resin. In particular, the addition of a freezing point depressing agent to a curative blend that results in a storage stable pigment dispersion would be advantageous to use in golf ball compositions.